Key runs under professional test champion (8)
I believe the answer is:
promoter
'champion' is the definition.
(thesaurus)
'key runs under professional test' is the wordplay.
'key' becomes 'e' (musical key).
'runs' becomes 'r' (cricket abbreviation).
'under' means one lot of letters go next to another (below, in a down clue).
'professional' becomes 'pro' (abbreviation).
'test' becomes 'MOT' (as in an MOT test for a vehicle).
'e'+'r'='er'
'pro'+'mot'='promot'
'er' after 'promot' is 'PROMOTER'.
(Other definitions for promoter that I've seen before include "Organiser (of a sporting event)" , "Sponsor" , "Impresario" , "One organising publicity" , "One putting on a sporting event" .)